Could you have some wear on the splines themselves?
Here's the part numbers for the seal and nut.
4210973 WASHER, Spline Seal Yoke, Rubber
4167924 NUT, Rear Yoke
Here's the part numbers for the seal and nut.
4210973 WASHER, Spline Seal Yoke, Rubber
4167924 NUT, Rear Yoke